The present invention relates in general to the production of petroleum from petroleum wells, and, in particular, to systems for providing clean power fluid for downwell machinery, such as the engine for pumps.
It is common practice to employ a power fluid to operate machinery downhole in the petroleum well. For example, power fluid can operate the engine of a downhole pump that pumps petroleum from a petroleum bearing formation to the surface. Typically, power fluid is pressurized at the surface to raise its head, as by a multiplex pump. Pressurized power fluid is then fed down into a tubing string of the well to the downhole machinery it operates. Exhaust power fluid is usually brought up the well with well fluid and the two together are called production fluid. Alternatively, power fluid can be maintained separate from well fluid and routed through its own tubing string.
Machinery handling the power fluid, including the downhole machinery, is, of course, sensitive to solid abrasives that are entrained in the production fluid stream leaving the well. Small solid particles in the power fluid can score and damage power fluid circuit machinery. For example, downhole seals of a pump engine can be lost, with a loss of the effectiveness of the engine producing the power for its pump. This loss also requires the lifting of the engine from the well for renewal or replacement. Production time is lost during engine shutdown, which time includes the time lost in raising and lowering the downhole machinery from the well. To get rid of the solid abrasives, the practice has been to separate solids from the power fluid before the power fluid is used again.
The problem of power fluid contamination with solid abrasives is a continuous problem regardless of whether power fluid is maintained in a separate loop or whether power fluid is mixed with well fluid. Accordingly, treatment at the surface to remove solids is continuous.
These are several ways that separation of solids from production fluid have taken place in the past. One approach used a centrifugal pitot cleaner. Centrifugal force separates the phases of the production fluid in a rotating rotor in accordance with the density of the phases. The heavier solid material leaves the rotor through nozzles. Oil may be taken from the rotor through a stationary pitot tap. This system can be used with cyclones that reduce the kinetic head of the fluid and permit solid materials to drop out of suspension.
A second approach used with cyclones or with the pitot separators includes the use of separation tanks or separators. A separator is a large vessel that receives the production fluid and permits the fluid to separate into its phases by gravity. Solids will settle out at the bottom. The next phase will be water, the next after that will be an oil phase, and, in an ullage space above the oil will be a final gaseous phase.
In separation vessels it is desirable to maintain a minimum quantity of power fluid in order to have an adequate reserve for power fluid taken from the vessel to accommodate fluctuations in demand. This minimum quantity of fluid assures adequate volume of fluid to maintain the contents of the tank quiescent, and therefore to avoid mixing the previously separated solids into the fluid. An adequate quantity of fluid in the separation vessel also helps to assure that the proper phase of the liquid is present at the power fluid pickup.
Different petroleum wells have different requirements. Some wells have a very high water content or "cut". When this is the case, water is usually used as a power fluid. Other wells have a high oil cut and for these wells oil is the power fluid. Some wells have a viscous oil that contains sand that is extremely difficult to remove. For these wells, water is the power fluid. To provide a separation vessel suitable for the requirements of different wells requires that different percentage of oil and water cut be accommodated. It is also highly desirable that the serparation vessel have attendant machinery that is as simple as possible.